1. Field of the Invention
The present invention relates to an imaging device that uses a phenomenon of avalanche electron multiplication, a method of driving an imaging device, and a method of manufacturing an imaging device.
2. Description of Related Art
Imaging devices that use a phenomenon of avalanche electron multiplication have been proposed in the related art. For example, as disclosed in JP-A-4-115575, there is a technique of causing electron multiplication by generating a high electric field from a photodiode to a read gate. In addition, as shown in JP-A-4-176721, there is a configuration in which a high electric field is applied in the middle of a transfer path.
In addition, as shown in JP-A-7-66379, there is a configuration in which a stacked layer of a stacking type image sensor is used as an electron multiplication layer. Moreover, a technique of performing electron acceleration or electron multiplication outside an image sensor has also been proposed. An image sensor in which an image intensifier and a CCD are combined or an image sensor, in which a backside illuminated type CCD is provided in an electron tube having a photoelectric surface disclosed in JP-A-6-243795 and electrons accelerated at high speed are injected into a silicon layer of the CCD, has been put to practical use as an image sensor for professional special imaging.
However, in those disclosed in JP-A-4-115575 and JP-A-7-176721, an image magnification ratio of electrons is several percent to several tens of percent and an extremely high voltage is required to increase the image magnification ratio in a read gate. As a result, it becomes difficult to secure the reliability, and deviation in a multiplication factor for every pixel occurs and a signal amplified due to machining deviation of the read gate is generated. Furthermore, in order to realize a high multiplication factor in multiplication in a transfer path, it is necessary to apply a high voltage difference between adjacent electrodes, which causes a problem in the reliability. However, in order to drop a voltage, it is necessary to perform multiplication driving in many transfer stages. Accordingly, a study on the configuration, such as providing an additional transfer portion or providing a ring-shaped transfer portion, was required. For this reason, in any configuration disclosed in the documents above, it was necessary to increase the chip area in order to secure the reliability and a high multiplication factor.
In the case of the configuration disclosed in JP-A-7-66379, the stacked layer cannot necessarily be formed on an ideally flat substrate, such as an electron tube. Accordingly, it is very difficult to form the stacked layer uniform and it is not put to practical use.
In the case of the configuration disclosed in JP-A-6-243795 in which a backside illuminated type CCD is provided in an electron tube formed with a photoelectric surface, an increase in volume cannot be avoided. In particular, it was difficult to provide the structure in imaging apparatuses for consumer use, such as a digital camera and a video camera.